1. Field of the Invention
The present invention relates to a technique of adaptive equalizing of radio signals such as a frequency modulation signal (hereinafter referred to as FM signal), and more specifically, to an adaptive equalizer with a function of activating and stopping an adaptive equalization processing.
2. Description of Related Art
In a radio broadcasting system such as an FM radio or a wireless communication system, a transmission signal is deteriorated due to multipath distortion or noises in a wireless transmission path. To that end, an equalizer for recovering the received transmission signal that is deteriorated after propagating through the wireless transmission path is used on the wireless receiver side. In general, a state of the wireless transmission path is uncertain on the receiver side. Further, a wireless receiver with high mobility should follow change in characteristics of the wireless transmission path. Thus, an adaptive equalizer capable of following the change in characteristics of the wireless transmission path has been used.
In general, the adaptive equalizer is configured by a digital filter (hereinafter referred to as adaptive filter) such as an IIR (Infinite Impulse Response) filter and an FIR (Finite Impulse Response) filter, and an adaptation algorithm such as an LMS (Least Mean Square) algorithm and an NLMS (Normalized LMS) algorithm has been employed to optimize filter coefficients.
To optimize the filter coefficients based on the adaptation algorithm, a reference signal for estimating characteristics of the wireless transmission path is necessary. As one conceivable method, a training signal with a known signal pattern is sent earlier than an information signal and used as a reference signal to determine filter characteristics. As another method, a blind equalization method has been well known. The blind equalization method generates a reference signal from a received signal and thus does not need to previously send a training signal. Examples of the blind equalization method include a CMA (Constant Modulus Algorithm).
The CMA is an algorithm in general, which sets a statistical quantity regarding the filter output signal such as an envelope of a filter output signal and higher-order statistical quantity thereof as an index, and updates the filter coefficients to approximate the index to a target value. The CMA is effective for a modulation system where a modulation signal has constant amplitude characteristics such as an FM (Frequency modulation) signal or PM (Phase modulation) signal.
INOUE (Japanese Unexamined Patent Application Publication No. 2006-287654) discloses an adaptive equalizer having a CMA blind adaptive equalization function. The adaptive equalizer disclosed in INOUE determines whether a condition that a received signal is deteriorated by an update of filter coefficients of an adaptive filter is fulfilled or not, and suppresses the operation of the adaptive filter if this condition is fulfilled. According to the technique of INOUE, the received signal is deteriorated based on the determination whether the overmodulation is produced or not, whether a field strength is the weak electrical field or not, whether a sudden change in electrical field is produced, and whether the switching is based on the diversity reception operation.
Further, INOUE discloses two specific methods for suppressing the operation of the adaptive filter. The first method is to stop updating the filter coefficients of the adaptive filter. According to this first method, the filter coefficient of the adaptive filter is fixed to a value just before the coefficient update has been prohibited. Accordingly, the adaptive filter continues the fixed equalization processing while keeping the state before the operation is suppressed. The second method is to directly supply the received signal to the detector by bypassing the adaptive filter. According to the second method, the equalization processing is not carried out at all.
The adaptive equalization based on the CMA can adaptively configure a filter having characteristics opposite to characteristics of a wireless transmission path unless a factor of signal deterioration such as multipath distortion in a wireless transmission path, an interfering wave, and noise hinders estimation of a modulation signal with constant amplitude from a received signal. However, if there is a factor of signal deterioration that hinders estimation of a modulation signal with constant amplitude from a received signal, adaptive equalization processing is no longer performed stably through the CMA.
For example, when input signal intensity is greatly changed at a small interval, it is impossible to make the correction error to converge even when the CMA approximates the correction error to 0, which may lead to unstable operations such as oscillations in the filter coefficients.
Accordingly, when there is such a factor of signal deterioration, it is preferable to control the equalization ability of the adaptive filter so that the operational stability of the adaptive filter is fully secured.
The above-described adaptive equalizer disclosed by INOUE only operates the adaptive filter as a fixed equalization filter or carries out the detection processing by bypassing the adaptive filter without carrying out the equalization processing after determining whether the condition that the received signal is deteriorated by the update of the filter coefficients is fulfilled or not. Accordingly, the adaptive equalization processing is not carried out at all after the determination of the satisfaction of the condition. In summary, the equalization ability control of the adaptive filter executed by the adaptive equalizer of INOUE alternatively selects one of the state in which the adaptive equalization is carried out and the state in which the adaptive equalization is not carried out at all (state in which the fixed equalization is performed or state in which the equalization processing is not performed) based on the satisfaction of the condition that the received signal is deteriorated.